Crystalline colloidal narrow band radiation filters were described in U.S. Pat. Nos. 4,627,689 and 4,632,517, patented Dec. 9, 1986 and Dec. 30, 1986, respectively, to Sanford A. Asher. In accordance with the inventions described therein, electrically charged particles were dispersed in a dielectric liquid to form a colloidal dispersion. The liquid dispersion was contained in a thin planar cell within walls of transparent material such as methacrylate plastic, coated glass or quartz.
The highly ordered crystalline arrangement of colloidal particles that is needed for narrow band diffraction was obtained through use of a combination of optimum conditions for making and containing the colloidal suspension. A narrow band of radiation, less than 20 nm in width, supposedly could be selectively diffracted by the colloidal array filters described by Asher. In some preferred embodiments, Asher stated that more than 99% of radiation in a band width of less than 5 nm could be diffracted while more than 80% of the intensity of adjacent wavelengths was transmitted.
In the crystalline colloidal arrays described by Asher, electrically charged microspheres of polystyrene were suspended in a dielectric liquid, preferably in water, and the electrical interaction between charged particles in the suspension formed the crystalline array of the particles. In thin layers of such colloidal suspensions, contained in thin cells within walls of plastic or coated glass or quartz, a highly ordered crystalline array was achieved. The high degree of ordering of polystyrene microspheres within the crystalline colloidal structure was achieved by careful cleaning of the microspheres and the liquid medium to remove electrolytes and surfactant impurities therefrom and by using microspheres of uniform particle size to obtain a monodisperse suspension.
The wavelength of the light diffracted in such colloidal arrays is determined by the lattice spacing in the crystalline colloidal structure which in turn is determined by the size of the dispersed particles, the concentration of the colloidal particles in the medium, and the thickness of the colloidal layer. The band width of the diffracted light is narrowed as the degree of crystalline order in the suspension is increased.
The major deficiency associated with the colloidal arrays disclosed by Asher is their fragility. The lattice of the array may be destroyed when subjected to shock, temperature variations and ionic influences. This deficiency renders the arrays useless in filter applications.
The Asher U.S. Pat. No. 4,627,689, mentioned above, also described a technique for making a filter in liquid medium with a concentration gradient, "and then `freezing` the nonuniform condition, e.g., by polymerization techniques." No further description of a filter embodying that structure, nor of any "polymerization technique" was described.
U.S. Pat. No. 4,451,412, patented May 29, 1984 to B. Loiseaux et al. describes a process for producing diffracting phase structures in a medium formed of microspheres of polystyrene suspended in a polymerizable liquid. An elementary holographic image was formed in the suspension by interference patterns. The image was then fixed by polymerization of the medium. For suspensions in organic liquids, the use of polymerizable acrylic monomers was described. For suspensions in aqueous medium, the use of water-soluble monomers, particularly acrylamide monomers, with diazo free radical initiators was described.
It has been found that a highly ordered crystalline array of polystyrene microspheres in a aqueous medium, as described by the Asher patents cited above, can not be fixed by the polymerization techniques described by the Loiseaux patent mentioned above. Because of the ionic properties of the diazo polymerization initiators described in U.S. Pat. No. 4,451,412 they prevent the formation of a highly ordered crystalline array in the liquid medium.